Attempts by people in China to mark the 21st anniversary of the Tian'anmen Square massacre by using the location-based service FourSquare to "check in" at it and leave messages have been blocked by the government there - which has cut off access to the site.

It suggests that the reason for the block is because Foursquare checkins are frequently fed through to Twitter and Facebook, both of which are widely used in China. That's as long as the censors are happy; Twitter has been blocked from time to time by Chinese censors.

But of course given that June 4 is the anniversary of the vicious repression of protests by students seeking democratic reforms in 1989, it has been watching internet sources for any sign reflecting the significance of the data.

The blog's author writes:

"because foursquare speaks to both Twitter and Facebook, some of us posted that onto — right… especially Twitter. The censors probably went, "Ah…" and boom — blocked the site outright.

"Jeepers. This is the stuff that must have the censors soiling in their pants. Virtually hundreds of people checked into Tian'anmen Square, the place where "something baaad happened" 21 years ago. Of course, the square itself is "safe" (in the real world) today, with cops even in helmets, as well as SWAT forces, all reported in the vicinity of the world's largest square.

"In place of student demonstrators and their banners, we have people basically filing into foursquare and leaving "sensitive comments" as "tips". Those checking in included folks outside Beijing; @isaac (Isaac Mao), @rejon (Jon Philips) and the rest just flocked to the square, even if they were based elsewhere. (You can do this, by the way, by going to the Foursquare mobile site or even the main site and change your location — although if you're not actually there, that could be seen as cheating…) "

The fact that it is easy to "check in" to a location on Fourqsuare without being physically present has given democracy protesters in China a new way to express their concern. But that has once again become a cat-and-mouse game with the censors - who, once more, have the upper hand.